Citrate Utilization Test- Principle, Media, Procedure and Result

This test is among a suite of IMViC Tests (Indole, Methyl-Red, Vogues-Proskauer, and Citrate) that are used to differentiate among the Gram-Negative bacilli in the family Enterobacteriaceae.

Principle of Citrate Utilization Test

Citrate agar is used to test an organism’s ability to utilize citrate as a source of energy. The medium contains citrate as the sole carbon source and inorganic ammonium salts (NH4H2PO4) as the sole source of nitrogen.

Bacteria that can grow on this medium produce an enzyme, citrate-permease, capable of converting citrate to pyruvate. Pyruvate can then enter the organism’s metabolic cycle for the production of energy. Growth is indicative of utilization of citrate, an intermediate metabolite in the Krebs cycle.

When the bacteria metabolize citrate, the ammonium salts are broken down to ammonia, which increases alkalinity. The shift in pH turns the bromthymol blue indicator in the medium from green to blue above pH 7.6.

Christensen developed an alternative citrate test medium that does not require the organism to use citrate as a sole carbon source. Christensen’s medium contains both peptone and cysteine. Thus citrate-negative bacteria can also grow on this medium. A positive reaction shows that the organism can use citrate but not necessarily as the sole carbon source.

Media used in Citrate Utilization Test

Simmon’s Citrate Agar

Composition

Sodium Chloride

5.0 gm

Sodium Citrate (dehydrate)

2.0 gm

Ammonium Dihydrogen Phosphate

1.0 gm

Dipotassium Phosphate

1.0 gm

Magnesium Sulfate (heptahydrate)

0.2 gm

Bromothymol Blue

0.08 gm

Agar

15.0 gm

Deionized water = 1,000 ml
Final pH 6.9 +/- 0.2 at 25 degrees C.

Preparation

Dissolve above salts in deionized water.

Adjust pH to 6.9.

Add agar and Bromothymol blue.

Gently heat, with mixing, to boiling until agar is dissolved.

Dispense 4.0 to 5.0 ml into 16-mm tubes.

Autoclave at 121 degree C under 15 psi pressure for 15 minutes.

Cool in slanted position (long slant, shallow butt).

Tubes should be stored in a refrigerator to ensure a shelf life of 6 to 8 weeks.

The uninoculated medium will be a deep forest green due to the pH of the sample and the bromothymol blue.

Procedure of Citrate Utilization Test

Streak the slant back and forth with a light inoculum picked from the center of a well-isolated colony.